1. Field of the Invention
This invention relates to a compact, portable, molecular drag vacuum pump. More particularly, the present invention relates to a miniature drag pump having a rotor with three surfaces for contacting the gas stream in order to develop high compression ratio and very low power consumption.
2. State of the Art
In recent years, miniature, portable, and ambulatory chemical and biological sensors have been developed. These sensors have many potential applications, such as in hand-held chemical analyzers, biological detection systems, and other portable sensory instruments. Such instruments may find advantageous use by soldiers to detect the presence of chemical and biological warfare agents, or as a simple and rapid means of on-site testing of environmental contaminants or unknown substances found at a crime scene.
However, to fully realize the benefit of these new ambulatory sensor systems, new, compact, low power vacuum pumps are needed. Existing vacuum pumps capable of achieving the desired pumping characteristics are typically too large and consume too much power for compatibility with portable sensor systems. Similarly, conventional pumps that are small enough for such applications generally cannot provide the high vacuum pressure required for highly accurate sensing and testing. Such pumps are generally ineffective in the Knudsen range, where the concentration of remaining gas molecules is too small for the pump to physically move. Several solutions to this type of problem have been tried, including cryogenics, absorption of remaining gas particles, and diaphragm pumps, but without adequate success.
One type of pump that is promising for application in this area is the molecular drag vacuum pump. The concept of the molecular drag pump was first introduced by Gaede in 1913, W. Gaede, Annals of Physics, vol. 41, 337 (1913), and was later applied in a disk shaped version by Siegbahn in 1944. M. Siegbahn, Archives of Mathematics, Astronomy, and Physics, vol. B30, 2 (1944). The basic principle of operation of the molecular drag pump is to transfer momentum from a high speed moving surface, such as a disk or drum, to the associated gas, to thereby compress and direct the gas toward an outlet port. Drag between the moving surface and the gas causes the average kinetic energy of the gas molecules to increase in the pumping direction, making the remaining gas more prone to evacuate the pump chamber. In the very low pressure range, this type of pump action causes a larger number of molecules to evacuate, resulting in a more complete vacuum.
Unfortunately, traditional molecular drag pumps do not make efficient use of the space available for pumping, and generally rely on oil lubricated bearings or large, power-hungry magnetic or air bearings to achieve the desired performance. In addition, in traditional molecular drag pumps, the performance is severely limited by the tolerance between the wiper and the rotor. To solve these problems it would be desirable to have a compact molecular drag pump that allows a reduction in the fabrication tolerances of the pump parts, yet provides the desired performance. It would also be desirable to have a compact molecular drag pump that either does not require oil lubrication, or makes use of efficient compact lubricated bearings. It would also be desirable to have a compact molecular drag pump which compresses the gas in a series of stages in order to sequentially increase the pressure. Finally, it would be desirable to have a multiple stage molecular drag pump which accommodates leakage between pumping stages by directing higher pressure leakage gas into the prior stage to prime the incoming stream.
It is therefore an advantage of the present invention to provide a compact, molecular drag pump that is suitable for use with portable or hand-held sensing and testing systems.
It is another advantage of this invention to provide a compact, molecular drag pump that is more efficient than conventional drag pumps.
It is another advantage of this invention to provide a compact, molecular drag pump that obtains larger compression ratios than traditional pumps of similar size.
It is another advantage of this invention to provide a compact, molecular drag pump that allows a reduction in the fabrication tolerances of the pump parts, yet provides the desired performance.
It is another advantage of this invention to provide a compact, molecular drag pump that makes use more efficient and compact bearings for moving components.
It is yet another advantage of this invention to provide a compact, molecular drag pump that has reduced power consumption compared to traditional drag pumps.
It is still another advantage of this invention to provide a compact, molecular drag pump which provides multiple surfaces on a moving rotor for contacting the gas in a series of stages, in order to sequentially increase the pressure.
It is also an advantage of this invention to provide a multiple stage molecular drag pump which accommodates leakage between pumping stages by directing higher pressure leakage gas into the prior stage to prime the incoming stream.
The above and other advantages are realized in a compact molecular drag vacuum pump for pumping a gas stream from an inlet to an outlet, comprising a spinning cylindrical disk having a channel formed in its edge and disposed within a housing. A plurality of circular passageways, preferably three, are formed on the inside of the housing, the first passageway being disposed adjacent to the top surface of the spinning disk, the second passageway being formed within the channel formed in the outer edge of the spinning disk, and the third passageway being disposed adjacent to the bottom surface of the spinning disk. A gas stream is introduced into the first passageway through an inlet and is compressed by contact with the spinning disk in successive stages in the first passageway, the second passageway, and the third passageway, then exits through an outlet from the third passageway. Conformable plastic wiper plates are disposed at the end of the first passageway and the second passageway, respectively, to direct the gas stream to successive stages of the pump, the wiper plates being configured to conform to the shape of the spinning disk so as to substantially reduce leakage therearound.
Some of the above advantages are also realized in a compact molecular drag pump as described, further comprising an integrally formed motor comprising permanent magnets disposed in the rotor and corresponding electromagnetic coils disposed in the housing, whereby electric current provided to the coils causes the rotor to spin. Other advantages and features of the present invention will be apparent to those skilled in the art, based on the following description, taken in combination with the accompanying drawings.